This application claims priority to Japanese Patent Application No. 2001-375892 filed on Dec. 10, 2001.
1. Field of the Invention
The present invention relates to a radar device. The invention particularly relates to a radar device mounted on a vehicle to be used for a collision alarm, for collision prevention, for auto-cruise control, and for automatic driving.
2. Description of the Related Art
Among radar devices that measure a relative speed between a vehicle and a target object or a distance between the vehicles, there are radar devices that employ various kinds of radar systems such as an FM-CW (Frequency Modulated-Continuous Waves) radar and a pulse Doppler radar. Particularly, the FM-CW radar device has advantages in that it has a relatively compact circuit structure and low cost, and that it is possible to obtain a distance between moving units such as vehicles and a relative speed between the vehicles at the same time. Therefore, this FM-CW radar device is currently employed in many vehicles.
FIG. 1 shows one example of a conventional FM-CW radar device.
In FIG. 1, a millimeter-wave signal generated by a voltage control oscillator (VCO) 3 is FM-modulated based on an FM signal from a modulation signal generating section 6. A triangular wave signal is generally used as the modulation signal. A triangular-wave FM-modulated transmission signal is emitted from a transmission antenna 1 to a forward moving unit such as a vehicle. A reception antenna 2 receives a reflection signal from the forward moving unit. An RF mixer 4 mixes the reception signal and a part of the transmission signal, and outputs a beat signal. A signal processing section 7 obtains a distance between the forward moving unit and the own vehicle and a relative speed between the two vehicles, by using frequency information included in the beat signal.
When a frequency of the transmission signal generated by the voltage control oscillator 3 is f0 and when a beat frequency of transmission/reception signals is fxcex4, a frequency of the reception signal received by the reception antenna 2 is expressed as f0+fxcex4. The RF mixer 4 mixes the reception signal of the frequency f0+fxcex4 and the transmission signal of the frequency f0, and outputs a beat signal having the frequency fxcex4 as a differential signal between the two signals.
A signal level of the signal from the voltage control oscillator 3 has a predetermined frequency characteristic within a range of the output frequency, and it is difficult to make the transmission power-frequency characteristic completely flat in the total transmission system.
FIG. 2 shows one example of variations in output signal levels d0 to d1 of the voltage control oscillator 3 within an output frequency range from f0 to f1. In this example, an output signal frequency is swept repetitively from f0 to f1 and from f1 to f0 based on a triangular wave base-band signal from the modulation signal generating section 6. During this period, an output signal level also changes repetitively from d0 to d1 and from d1 to d0.
As a result, a transmission signal is AM modulated in synchronism with an FM modulation timing according to the triangular wave base-band signal as a modulation signal. When the RF mixer 4 uses a part of the transmission signal as a local signal, the RF mixer 4 generates noise of a low frequency area that includes noise due to the AM modulation, in the output of the RF mixer 4 according to its AM demodulation function. This noise component will hereinafter be referred to as xe2x80x9cFMAM noisexe2x80x9d.
A high-pass filter (HPF) 5, as shown in FIG. 1, is inserted in a beat signal processing system in order to remove this FMAM noise component. In place of this high-pass filter 5, a band-pass filter or the like may be used.
However, when the high-pass filter 5 is used according to the conventional practice, a signal component (a short-distance beat signal component) existing in the low frequency area in which the FMAM noise also exists, is attenuated at the same time. Therefore, there has been a problem that the signal detection sensitivity in the low frequency area is lowered. There has also been a possibility that when an FMAM noise level changes due to temperature and other factors, this change of the noise level is erroneously detected as a signal.
In the light of the above problems, it is an object of the present invention to provide a radar device that has a unit for removing the FMAM noise without lowering the signal detection sensitivity.
It is another object of the present invention to provide a radar device that can obtain satisfactory signal detection sensitivity within a range from a short distance to a long distance by suitably controlling the FMAM-noise removing unit.
According to one aspect of the present invention, there is provided a radar device that transmits a frequency modulation signal by switching the frequency modulation signal using a first switching signal, receives a signal reflected from a target object, switches the reception signal using a second switching signal, mixes the switched reception signal with the transmission signal, and further mixes the mixed signals using a third switching signal thereby to obtain a beat signal. The radar device obtains a distance from the target object and a relative speed of the target object from the beat signal. The second switching signal and the third switching signal have the same switching frequency, and have a predetermined phase difference between the phases of the two switching signals to cancel and remove an amplitude modulation component included in the frequency modulation signal. The predetermined phase difference is substantially 90 degrees. Based on this, it is possible to obtain a target radar device that lowers the FMAM noise.
According to another aspect of the invention, the ON time of the second switching signal is at least two times the time from when a transmission wave is transmitted and reflected from the target object located at a maximum detection distance till when this reflected wave returns to the transmission origin. Based on this, it is possible to carry out a design that makes it possible to detect up to the maximum detection distance. Within this range, it is possible to set both the ON time of the first switching signal and the ON time of the second switching signal, or the ON time of the first switching signal or the ON time of the second switching signal, shorter than the time required for a detection distance. Further, it is possible to change the frequencies of the first, second and third switching signals corresponding to the detection distance respectively.
For example, at the time of measuring a short-distance signal, it is possible to set the ON time of the first switching signal shorter than the ON time of the second switching signal, from the viewpoint of the transmission signal (refer to FIG. 8 to be explained later). On the other hand, from the viewpoint of the reception signal, it is possible to set the ON time of the second switching signal shorter than the ON time of the first switching signal. In both cases, it is possible to improve the reception level by increasing the frequencies of the first and second switching signals.
In order to simplify the circuit, the first switching signal and the second switching signal have the same switching frequency, and these switching signals are switched on in mutually opposite phases. These switching signals are rectangular waves having a 50% duty ratio. Further, with a view to improving the cancellation rate of the FMAM noise, the first, second and third switching signals are generated in the same oscillation source, in order to maintain accurate synchronization and an accurate phase difference. Further, there is provided a unit for adjusting a delay between the first, second and third switching signals respectively.
According to still another aspect of the present invention, there is provided a phase variable unit that can change a phase difference between the phase of the second switching signal and the phase of the third switching signal. The phase difference is changed to either 0 degree or 90 degrees. With this arrangement, it is possible to carry out a long-distance measuring based on a phase difference of 0 degree and a short-distance measuring based on a phase difference of 90 degrees, substantially at the same time. This changeover is carried out at each half period or at each integer times one period of the frequency modulation signal. This period corresponds to a measuring operation period of the FM-CW radar, and has an advantage in that this facilitates the signal processing.
Further, the radar device has a comparing and detecting unit that compares measurement information based on a phase difference of 0 degree or a phase difference of 90 degrees according to the phase variable unit, and detects in high precision a distance from the target object and a relative speed. The comparing and detecting unit uses spectrum information of a beat signal as the measurement information based on a phase difference of 0 degree or a phase difference of 90 degrees, and corrects one spectrum information based on the other spectrum information. Based on this, it becomes possible to carry out an accurate extraction of a beat signal at a short distance close to the FMAM noise, and a fault diagnosis and a correction of noise filter characteristics at a short distance or a long distance.
Further, the phase variable unit may be composed of a plurality of mixing systems that use a plurality of the third switching signals having mutually different phase differences between the third switching signals and the second switching signal respectively. Based on this, it becomes possible to carry out both a short-distance measuring and a long-distance measuring at the same time. In actual practice, a digital signal processor (DSP) or a microprocessor unit (MPU) carries out a signal processing to execute the processing of the mixing systems. One of the mixing systems is selected in sequence to carry out the processing.